Renewable energy based Distributed Generators (DGs), such as photovoltaic solar and wind generators, are receiving strong encouragement globally through various incentive programs. Solar farms and wind farms which generate power from few kW to several hundred MW are being installed in both distribution and transmission systems. Distributed generation power sources connected at one or more locations within the distribution system have brought new issues and problems to existing power systems.
One of the major obstacles in existing electric power systems is that the integration of more distributed generators (DGs) to the network increases the short circuit level significantly due to the contribution of the DG to the fault. In general all forms of DG contribute some increase to fault levels. The connection of DGs to the distribution network could therefore result in fault levels exceeding the design limit of the network, particularly if the network is already operating close to its design limit (i.e., with low fault level headroom). When fault level design limits are exceeded, there is a risk of damage to and failure of the equipment with consequent risk of injury to personnel and interruption of supply under short circuit fault conditions.
Faults (short circuits) are inevitable. Any power system is expected to suffer several faults each year. The number will depend on exposure to lightning and damage from trees, as well as the age of the system's components. When a short circuit fault occurs in the distribution network, a short circuit current will flow to the fault location. This short circuit current is detected and cleared by existing protection equipment, such as circuit breakers or fuses.
However, when fault levels go beyond the existing design limits due to the connection of DGs, uprating the capability of existing protection equipment such as circuit breakers is the only option to increase the fault level capabilities of the network. It is likely that a large area of the network must be reworked in such cases, making this an exorbitantly expensive solution, particularly if transformers and cables or overhead lines are also involved. Hence, utility companies are limiting the connection of DGs into their existing network, resulting in a loss of opportunity to integrate more renewable energy generation into the transmission and distribution grid system.
Even though inverter based DGs, such as PV solar farms, contribute far less short circuit current to the network compared to conventional synchronous generators, the short circuit contribution is nevertheless considered unacceptable by many utility companies as it may potentially damage their transformers and circuit breakers, especially if there are several DGs operating together.
Moreover, according to industry standards (e.g., IEEE-1547 or UL-1741) regardless of fault level, DGs are required to disconnect upon detection of fault on the system. Conventional fault detection techniques based on over-voltage, under-voltage and over-current signals, which are used to operate the protective circuit breakers and disconnect the DGs from the network, are fast but yet not adequate to meet the stringent requirement of utilities. Even a small contribution of short circuit current may unacceptably overload the circuit breakers. This means that the detection of faults and disconnection of DGs from the network should be done as quickly as possible.
In light of the above, there is a need for solutions which mitigate if not overcome the shortcomings of the prior art.